1. Field of the Invention
This invention relates to an element suitable for carrying out the temperature sensing by the changes of resistivity with changes of temperature (said element is referred to as "thermistor" hereinafter) and more particularly it relates to a thermistor composition capable of providing a thermistor in which a resistivity in the range of 1.OMEGA.-1M.OMEGA. can be obtained, dispersion of resistivity is within .+-.15%, voltage-dependence of resistivity, namely, varistor constant is 1.00-1.05, current noise is 2% or less and resistivity aging characteristic is 2% or less, and furthermore, this invention relates to a thick film thermistor prepared using said composition.
2. Description of the Prior Art
Heretofore, thick film thermistors have been produced, in the same manner as in the production of general thick film elements, by preparing a thermistor paste from a mixture of a complex oxide powder of traditional metals such as nickel, manganese, cobalt and the like which has thermistor characteristic, and shows change of resistivity with changes of temperature (said powder will be referred to as "thermistor characteristic powder" hereinafter), a glass frit and an organic vehicle. Said thick film thermistors, however, have various defects, namely, (1) low resistivity of less than 1 k.OMEGA. cannot be obtained, (2) resistivity greatly changes with changes of thickness of thermistor film and dispersion of resistivity is very large, i.e., .+-.30%, (3) voltage-dependence of resistivity is high, i.e., varistor constant is about 1.10, (4) current noise is high, i.e., about 10%, (5) resistivity aging characteristic is more than 5%, etc. Thus, thick film thermistor excellent in aging characteristic and high in accuracy cannot be obtained unless all of these defects have been removed.
As the result of researches on various materials, the said defects (1)-(4) have been eliminated by the addition of noble metal powder, especially, silver and palladium as the third component, but it is still necessary to search for the materials capable of improving the resistivity aging characteristic as mentioned in the above (5).